<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width" />
    <title>带交互功能的HTML5+JS烟花特效</title>
    <title></title>
    <style>
        /* basic styles for black background and crosshair cursor */

        body {
            background: #000;
            margin: 0;
        }
        canvas {
            cursor: crosshair;
            display: block;
        }
    </style>
</head>
<body>
<canvas id="canvas">Canvas is not supported in your browser.</canvas>
</body>
</html>

<script>
    // when animating on canvas, it is best to use requestAnimationFrame instead of setTimeout or setInterval
    // not supported in all browsers though and sometimes needs a prefix, so we need a shim
    window.requestAnimFrame = (function() {
        return window.requestAnimationFrame ||
                window.webkitRequestAnimationFrame ||
                window.mozRequestAnimationFrame ||
                function(callback) {
                    window.setTimeout(callback, 1000 / 60);
                };
    })();

    // now we will setup our basic variables for the demo
    var canvas = document.getElementById('canvas'),
            ctx = canvas.getContext('2d'),
    // full screen dimensions
            cw = window.innerWidth,
            ch = window.innerHeight,
    // firework collection
            fireworks = [],
    // particle collection
            particles = [],
    // starting hue
            hue = 120,
    // when launching fireworks with a click, too many get launched at once without a limiter, one launch per 5 loop ticks
            limiterTotal = 5,
            limiterTick = 0,
    // this will time the auto launches of fireworks, one launch per 80 loop ticks
            timerTotal = 80,
            timerTick = 0,
            mousedown = false,
    // mouse x coordinate,
            mx,
    // mouse y coordinate
            my;

    // set canvas dimensions
    canvas.width = cw;
    canvas.height = ch;

    // now we are going to setup our function placeholders for the entire demo

    // get a random number within a range
    function random(min, max) {
        return Math.random() * (max - min) + min;
    }

    // calculate the distance between two points
    function calculateDistance(p1x, p1y, p2x, p2y) {
        var xDistance = p1x - p2x,
                yDistance = p1y - p2y;
        return Math.sqrt(Math.pow(xDistance, 2) + Math.pow(yDistance, 2));
    }

    // create firework
    function Firework(sx, sy, tx, ty) {
        // actual coordinates
        this.x = sx;
        this.y = sy;
        // starting coordinates
        this.sx = sx;
        this.sy = sy;
        // target coordinates
        this.tx = tx;
        this.ty = ty;
        // distance from starting point to target
        this.distanceToTarget = calculateDistance(sx, sy, tx, ty);
        this.distanceTraveled = 0;
        // track the past coordinates of each firework to create a trail effect, increase the coordinate count to create more prominent trails
        this.coordinates = [];
        this.coordinateCount = 3;
        // populate initial coordinate collection with the current coordinates
        while (this.coordinateCount--) {
            this.coordinates.push([this.x, this.y]);
        }
        this.angle = Math.atan2(ty - sy, tx - sx);
        this.speed = 2;
        this.acceleration = 1.05;
        this.brightness = random(50, 70);
        // circle target indicator radius
        this.targetRadius = 1;
    }

    // update firework
    Firework.prototype.update = function(index) {
        // remove last item in coordinates array
        this.coordinates.pop();
        // add current coordinates to the start of the array
        this.coordinates.unshift([this.x, this.y]);

        // cycle the circle target indicator radius
        if (this.targetRadius < 8) {
            this.targetRadius += 0.3;
        } else {
            this.targetRadius = 1;
        }

        // speed up the firework
        this.speed *= this.acceleration;

        // get the current velocities based on angle and speed
        var vx = Math.cos(this.angle) * this.speed,
                vy = Math.sin(this.angle) * this.speed;
        // how far will the firework have traveled with velocities applied?
        this.distanceTraveled = calculateDistance(this.sx, this.sy, this.x + vx, this.y + vy);

        // if the distance traveled, including velocities, is greater than the initial distance to the target, then the target has been reached
        if (this.distanceTraveled >= this.distanceToTarget) {
            createParticles(this.tx, this.ty);
            // remove the firework, use the index passed into the update function to determine which to remove
            fireworks.splice(index, 1);
        } else {
            // target not reached, keep traveling
            this.x += vx;
            this.y += vy;
        }
    }

    // draw firework
    Firework.prototype.draw = function() {
        ctx.beginPath();
        // move to the last tracked coordinate in the set, then draw a line to the current x and y
        ctx.moveTo(this.coordinates[this.coordinates.length - 1][0], this.coordinates[this.coordinates.length - 1][1]);
        ctx.lineTo(this.x, this.y);
        ctx.strokeStyle = 'hsl(' + hue + ', 100%, ' + this.brightness + '%)';
        ctx.stroke();

        ctx.beginPath();
        // draw the target for this firework with a pulsing circle
        ctx.arc(this.tx, this.ty, this.targetRadius, 0, Math.PI * 2);
        ctx.stroke();
    }

    // create particle
    function Particle(x, y) {
        this.x = x;
        this.y = y;
        // track the past coordinates of each particle to create a trail effect, increase the coordinate count to create more prominent trails
        this.coordinates = [];
        this.coordinateCount = 5;
        while (this.coordinateCount--) {
            this.coordinates.push([this.x, this.y]);
        }
        // set a random angle in all possible directions, in radians
        this.angle = random(0, Math.PI * 2);
        this.speed = random(1, 10);
        // friction will slow the particle down
        this.friction = 0.95;
        // gravity will be applied and pull the particle down
        this.gravity = 1;
        // set the hue to a random number +-20 of the overall hue variable
        this.hue = random(hue - 20, hue + 20);
        this.brightness = random(50, 80);
        this.alpha = 1;
        // set how fast the particle fades out
        this.decay = random(0.015, 0.03);
    }

    // update particle
    Particle.prototype.update = function(index) {
        // remove last item in coordinates array
        this.coordinates.pop();
        // add current coordinates to the start of the array
        this.coordinates.unshift([this.x, this.y]);
        // slow down the particle
        this.speed *= this.friction;
        // apply velocity
        this.x += Math.cos(this.angle) * this.speed;
        this.y += Math.sin(this.angle) * this.speed + this.gravity;
        // fade out the particle
        this.alpha -= this.decay;

        // remove the particle once the alpha is low enough, based on the passed in index
        if (this.alpha <= this.decay) {
            particles.splice(index, 1);
        }
    }

    // draw particle
    Particle.prototype.draw = function() {
        ctx.beginPath();
        // move to the last tracked coordinates in the set, then draw a line to the current x and y
        ctx.moveTo(this.coordinates[this.coordinates.length - 1][0], this.coordinates[this.coordinates.length - 1][1]);
        ctx.lineTo(this.x, this.y);
        ctx.strokeStyle = 'hsla(' + this.hue + ', 100%, ' + this.brightness + '%, ' + this.alpha + ')';
        ctx.stroke();
    }

    // create particle group/explosion
    function createParticles(x, y) {
        // increase the particle count for a bigger explosion, beware of the canvas performance hit with the increased particles though
        var particleCount = 30;
        while (particleCount--) {
            particles.push(new Particle(x, y));
        }
    }

    // main demo loop
    function loop() {
        // this function will run endlessly with requestAnimationFrame
        requestAnimFrame(loop);

        // increase the hue to get different colored fireworks over time
        hue += 0.5;

        // normally, clearRect() would be used to clear the canvas
        // we want to create a trailing effect though
        // setting the composite operation to destination-out will allow us to clear the canvas at a specific opacity, rather than wiping it entirely
        ctx.globalCompositeOperation = 'destination-out';
        // decrease the alpha property to create more prominent trails
        ctx.fillStyle = 'rgba(0, 0, 0, 0.5)';
        ctx.fillRect(0, 0, cw, ch);
        // change the composite operation back to our main mode
        // lighter creates bright highlight points as the fireworks and particles overlap each other
        ctx.globalCompositeOperation = 'lighter';

        // loop over each firework, draw it, update it
        var i = fireworks.length;
        while (i--) {
            fireworks[i].draw();
            fireworks[i].update(i);
        }

        // loop over each particle, draw it, update it
        var i = particles.length;
        while (i--) {
            particles[i].draw();
            particles[i].update(i);
        }

        // launch fireworks automatically to random coordinates, when the mouse isn't down
        if (timerTick >= timerTotal) {
            if (!mousedown) {
                // start the firework at the bottom middle of the screen, then set the random target coordinates, the random y coordinates will be set within the range of the top half of the screen
                fireworks.push(new Firework(cw / 2, ch, random(0, cw), random(0, ch / 2)));
                timerTick = 0;
            }
        } else {
            timerTick++;
        }

        // limit the rate at which fireworks get launched when mouse is down
        if (limiterTick >= limiterTotal) {
            if (mousedown) {
                // start the firework at the bottom middle of the screen, then set the current mouse coordinates as the target
                fireworks.push(new Firework(cw / 2, ch, mx, my));
                limiterTick = 0;
            }
        } else {
            limiterTick++;
        }
    }

    // mouse event bindings
    // update the mouse coordinates on mousemove
    canvas.addEventListener('mousemove', function(e) {
        mx = e.pageX - canvas.offsetLeft;
        my = e.pageY - canvas.offsetTop;
    });

    // toggle mousedown state and prevent canvas from being selected
    canvas.addEventListener('mousedown', function(e) {
        e.preventDefault();
        mousedown = true;
    });

    canvas.addEventListener('mouseup', function(e) {
        e.preventDefault();
        mousedown = false;
    });

    // once the window loads, we are ready for some fireworks!
    window.onload = loop;
</script>